1. Field of the Invention
This invention relates in general to motor control systems, and relates more particularly to control systems for polyphase brushless DC motors.
2. Description of the Prior Art
The use of polyphase brushless motors is well known in a number of areas, particularly in the disk drive area where such motors are commonly used to rotate the spindle which carries the recording disk or disks.
One example of such use is shown in European Patent Application 0251785, published Jul. 1, 1988 and entitled "Electric Motor Control Method and Apparatus." That publication discloses a three-phase brushless DC motor control system in which the disk drive motor is operated in three separate stages. During the first or startup stage, short bursts of current are applied to each phase winding of the motor, and the motor current conducted in response thereto in each phase winding is measured to provide an indication of the rotational position of the rotor relative to the three phase windings to determine to which phase winding the next torque-producing current is to be applied.
After the motor reaches a predetermined speed, the motor control enters the second stage in which the back EMF of the motor and the motor current are continuously monitored as a measure of motor speed until the desired rotational speed is reached. Thereafter, in the final stage, torque producing current is sequentially applied to each phase winding in synchronization with the rotor position by deriving positional information from one of the rotating disks itself.